Electrochromism relates to the reversible color change induced in a material by an applied electric potential or current. Materials that exhibit electrochromism are used in electrochromic windows, variable reflectance mirrors, and other types of electrochromic devices.
Electrochromic devices have been described, for example, in U.S. Pat. No. 3,451,741 to Manos; in U.S. Pat. No. 4,090,782 to Bredfeldt et al.; in U.S. Pat. No. 4,093,358 to Shattuck and Sincerbox; and in U.S. Pat. No. 4,139,276 to Clecak et al. An electrochromic device of the type described in these patents generally includes an electrolyte solution of a solvent, a soluble oxidant and a soluble reductant. The device also includes a transparent chamber for the solution and an anode and cathode in contact with the solution. When a DC potential is applied between the anode and the cathode, the reductant is oxidized, the oxidant is reduced, and the solution changes color. The oxidant and reductant react only with each other and when the reaction products do not degrade, the color change can be made reversible.
In recent years, highly conductive electrolyte salts that are molten at room temperature have been developed for electrochromic windows, variable reflectance mirrors, batteries, capacitors, and other important devices. Preferred anions of these “molten salts” include trifluoromethylsulfonate (CF3SO3−), bis(trifluoromethylsulfonyl)imide ((CF3SO2)2N−), bis(perfluoroethylsulfonyl)imide ((CF3CF2SO2)2N−) and tris(trifluoromethylsulfonyl)methide ((CF3SO2)3C−). Preferred cations include quarternary ammonium cations (see J. Sun et al., “Room Temperature Molten Salts Based on the Quarternary Ammonium Ion,” J. Phys. Chem. B, 1998, vol. 102, pp. 8858–8864, incorporated by reference), lithium (see Kirk-Othmer's Encyclopedia of Chemical Technology, Fourth Edition, 3, 1016–1018 (1992) and 1107–1109; and 15, 446–447 (1995) all incorporated by reference), and phosphonium. Preferred quarternary ammonium salts include tetraalkylammonium, pyridinium, pyridazinium, pyrimidinium, pyrazinium, imidazolium, pyrazolium, thiazolium, oxazolium, and triazolium (see U.S. Pat. No. 5,827,602 to V. R. Koch et al. entitled “Hydrophobic Ionic Liquids,” which issued Oct. 27, 1998, incorporated by reference herein).
Commercially available electrochromic windows and mirrors are generally limited to operating temperatures above 0° C. The molten salts described above have a much wider potential window, sometimes greater than 5 Volts, and low glass transition temperatures (some as low as −80° C.) or melting points (some below −20° C.). By replacing these molten salts for currently used solvents (propylene carbonate, for example), the environmental conditions for which electrochromic devices operate would be greatly extended. For these reasons, electrochromic oxidants, electrochromic reductants, and their electrochromic products that are all highly soluble in these molten salts are highly sought after materials.
In recent years, an important class of oxidants from the family of materials known as the viologens has emerged. An example of the dicationic 4,4′-bipyridinium portion of a viologen is shown below.
Examples of this viologen are methyl viologen (R=methyl), ethyl viologen (R=ethyl), and benzyl viologen (R=benzyl). These viologens have been isolated as salts of the chloride (Cl−), acetate (CH3CO2−) tetraflurorborate (BF4−), perchlorate (ClO4−), triflurormethanesulfonate (CF3SO3−), tetraphenylborate (BPh4−), and hexaflurophosphate (PF6−), to name a few (see, for example, U.S. Pat. No. 5,140,455 to D. V. Varaprasad et al. entitled “High Performance Electrochemichromic Solutions and Devices Thereof,” which issued Aug. 19, 1992; U.S. Pat. No. 5,239,405 to D. V. Varaprasad et al. entitled “Electrochemichromic Solutions, Processes for Preparing and Using the Same, and Devices Manufactured With the Same,” which issued Aug. 24, 1993; U.S. Pat. No. 5,500,760 to D. V. Varaprasad et al. entitled Electrochemichromic Solutions, Processes for Preparing and Using the Same, and Devices Manufactured With the Same,” which issued Mar. 19, 1996; U.S. Pat. No. 5,998,617 to R. Srinvasa et al. entitled “Electrochromic Compounds,” which issued Dec. 7, 1999; and U.S. Pat. No. 6,037,471 to R. Srinvasa entitled “Electrochromic Compounds,” which issued Mar. 14, 2000, all hereby incorporated by reference). While the viologen salts described in these patents have reportedly been used in combination with molten salts to provide electrolyte solutions for electrochromic devices, at least some of the viologen salts, the dichloride salts or their electrochemically generated reduction products, for example, may have limited solubility in molten salts and/or tend to form precipitates. Anions such as BF4− and PF6− may be unstable after long periods of time, and ClO4− containing salts are known to be unstable. These problems impede the performance of, and affect the lifetime of, the device. Thus, there remains a need for electrochromic salts that are highly soluble in molten salts.
Therefore, an object of the present invention is to provide electrochromic salts that are highly soluble in molten salts.
Another object of the present invention is to provide an electrolyte solution of a molten salt and highly soluble electrochromic salts.
Another object of the present invention is to provide an electrochromic device that employs an electrolyte solution of a molten salt and highly soluble electrochromic salts.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.